Analogue correlator with constant signal-to-noise ratio

ABSTRACT

An improved analogue correlator wherein the output of a multiplier is fed into a nonlinear amplifier having a feedback network which may contain a capacitor and in which the impedance and time constant increases for an output that is decreasing, thereby increasing the gain of the amplifier while also increasing the time constant of the amplifier by a second order in the presence of the capacitor. Because the time constant of the amplifier increases faster than the signal-to-noise input decreases, the signal-to-noise output from the amplifier gets better for low level signals and can be adjusted to maintain a substantially constant ratio over a relatively large range of input signals.

United States Patent Inventor Gaines M. Crook Canoga Park, Calif. Appl. No. 857,273 Filed Aug. 22, 1969 Continuation-impart of application Ser. No. 607,432, Jan. 5, 1967, now abandoned. Patented Feb. 23, 1971 Assignee TRW Inc.

Redondo Beach, Calif.

ANALOG CORRELATOR WITH CONSTANT Primary Examiner-Malcolm A. Morrison Assistant Examiner-Felix D. Gruber Att0meys Daniel T. Anderson, Edwin A. Oser and Jerry A.

Dinardo ABSTRACT: An improved analog correlator wherein the output of a multiplier is fed into a nonlinear amplifier having a feedback network which may contain a capacitor and in which the impedance and time constant increases for an output that is decreasing, thereby increasing the gain of the amplifier while also increasing the time constant of the amplifier by a second order in the presence of the capacitor. Because the time constant of the amplifier increases faster than the signalto-noise input decreases, the signal-to-noise output from the amplifier gets better for low level signals and can be adjusted to maintain a substantially constant ratio over a relatively large range of input signals.

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INVENTOR.

ANALOG CORRELATOR WITH CONSTANT SIGNAL- TO-NOISE RATIO CROSS-REFERENCE TO RELATED APPLICATION The present application is a continuation-in-part of my prior copending application Ser. No. 607,432, filed on Jan. 5, 1967, now abandoned, and assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION This invention relates generally to analogue correlators and more particularly to an analogue correlator having a substantially constant signal-to-noise ratio.

An analogue correlator is a device which compares two signals to determine whether the signals are from a common source. In the field of radio astronomy, for example, it is necessary to examine various noise signals and determine which signals from two different antennae are from a common source when each signal is composed of noise" in the presence of other and more intense noise. Here, it is required to determine whether two noise signals or voltages are derived from the same source. If they are not, they have zero crosscorrelation. When the input signal is very noisy, more band width or more time is required for smoothing if a usable output is to be produced. Since the output for the correlator decreases as the input signal decreases, the output signal becomes harder and harder to utilize. It is advantageous to examine certain phenomena which occurs in a noisy environment as fast as possible. If the time constant of the correlator were set so that the output signal is usable at poor signal-tonoise ratios, then the signal rates may be too slow at a good signal-to-noise ratio. If the time constant is set so that the desired data rate is obtained with good signals, then poor signals may be lost altogether. It is therefore highly desirable to have a correlator wherein for a fast data rate good signal-tonoise ratios are obtainable and usable data is still obtainable at a slower rate with poorer signal-to-noise ratios.

SUMMARY A signal correlator in accordance with the present invention provides a substantially constant signal-to-noise ratio of two noiselike input signals to be correlated. This makes it possible to determine, for example, whether the two signals have a common origin. The correlator includes a multiplier and a nonlinear operational amplifier coupled in series. The multiplier is responsive to the two input signals to be correlated and provides a multiplied signal. The operational amplifier provides an output signal. This operational amplifier has a feedback path including a nonlinear impedance element. This makes it possible to feed back a portion of the output signal to the input of the operational amplifier. As a result, the gain of the operational amplifier varies inversely proportional to the magnitude of the output signal.

It is also feasible to include a capacitor in parallel with the nonlinear impedance element in the feedback path of the operational amplifier. As a result the time constant of the amplifier varies also inversely with the magnitude of the output signal.

It is therefore an object of the present invention to providea correlator having a substantially constant signal-to-noise ratio.

It is another object of the present invention to provide an amplifier for a correlator having a time constant which varies as a function of the input signal level.

These and other objects of the invention will become more apparent when taken in conjunction with the following description and drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic,-partially in block form, illustrating the preferred embodiment of the invention.

FIG. 2 is a graph of the output voltage in millivolts as a function of time and illustrating the characteristics of a correlator having a constant gain and time constant.

FIG. 3 is a chart of the gain in db as a function of time and illustrating one of the characteristics of the embodiment of FIG. 1.

FIG. 4 is a chart of the output voltage in millivolts as a function of time and illustrating a second characteristic of the embodiment of FIG. 1.

Referring to FIG. 1, a multiplier 10 is provided with two input signal terminals A and B. A third terminal 12 provides a reference and is connected to ground. The multiplier 10 may be any one of the well known types of multiplier circuits for multiplying two electrical signals and providing a multiplied output signal. The output of the multiplier 10 is fed to an operational amplifier 11 via a resistance R. The operational amplifier 11 is a conventional operational DC amplifier which is provided with a feedback network 14. The feedback network may be comprised of a capacitor C connected from the output terminal c to the input of the operational amplifier, and includes a plurality of rectifying diodes Rdl, serially connected, providing a first unidirectional current path from the output terminal e to the input of the operational amplifier and a second plurality of rectifying diodes Rd2, providing a second unidirectional current path in the opposite direction from the input of the amplifier to the output terminal e An output reference terminal 13 is connected to ground which also provides a reference point for the operational amplifier 11. The unidirectional current path of diodes Rd2 provides the amplifier with symmetrical operation which is necessary when monitoring correlated signals because the correlation function may have negative values.

The output signal-to-noise ratio is:

v f where Af is the input band width in cycles per second I is the integration time in seconds S/N is the input signal-to-noise ratio If the input signal is very noisy, then more band width or more time t is required for smoothing if a usable output is to be produced. The serially connected diodes Rdl and Rd2, provide a nonlinear resistance in the feedback path of the amplifier. The forward biased diodes exhibit a range of nonlinear resistance in the forward conduction region. Over a portion of the range of this region, the transfer characteristics of these devices approximates a logarithmic function of voltage vs. current. The transfer characteristics of junction-type transistors and diodes is logarithmic in the region where the currents involved are substantially large as compared to the leakage currents and where the series ohmic resistance is low as compared to their dynamic resistance. The gain of the operational amplifier will therefore vary as a function of the nonlinear resistance displayed by the feedback diodes. Then for a low level signal, the gain of the amplifier will be a maximum and for a high level signal, the gain will be a minimum and the variation in the gain between these two points will be nonlinear. By placing the capacitor C in the feedback path, which is optional, it is possible to achieve a variable time constant for the amplifier. The time constant of the signal output is given by t= AR,C, where t= time constant A signal gain R dynamic forward resistance of diodes C capacitance of the feedback capacitor It can be seen from the above equation that as the value of R, changes, the value of t will change, thereby varying the time constant of the amplifier. Since a change of R, also changes the gain, A, of the stage, the time constant t changes by a second order.

In operation, as the signal-to-noise output decreases by an order of magnitude, the'following will occur: the output of the amplifier will tend to drop to approximately one-tenth of its former value; the gain A increases by a factor of something less than 10, say, approximately 4; the feedback resistance (dynamic resistance of the diodes) increases by the same factor and the time constant 2 increases by a factor of 4 X 4 or 16.

Since the value of t increases faster than the input signal to noise ratio decreases, the output signal-to-noise out gets better within the limits of the dynamic range of the system.

The two input signals labeled A and B in FIG. 1 are the two signals to be correlated. These two signals may be noiselike signals, that is, signals with a relatively large noise, while the actual signal may be much smaller in magnitude. The analogue signal correlator of the invention may be used for correlating two signals to determine whether they originate from a common source. Therefore, in general, it may not be desired to compare the same signal with another portion of the same signal at a different time. Accordingly, it will simply be assumed that the two signals are to be compared, one with the other at some predetermined time. This is the reason why in general there is no need to include a time delay between the multiplier 16) and one of the input leads of the signals A or B. Therefore the correlator operates as if the two signals were in phase at the predetermined time. The uses of such an invention will be pointed out hereinafter.

FIG. 2 shows a representation of a chart recording plotting the output voltage in millivolts and taken at the output of a correlator which has a constant time constant and constant gain. This chart is taken at a constant paper speed of seconds per inch of travel. The input levels (with Odb 0.77 V. are shown at the points which the new level is switched in. Since the correlator has a characteristic settling time, this is displayed by the curvature of the presentation in the knee of the graph after each change of input. This presentation has the advantage of being able to display the output signal level, the rate of change of output and the relative noisiness of the signal. It was also used in obtaining the graphs of FIGS. 3 and 4.

The time scale on the plot has nothing to do with the time constant of the correlator except as the time constant charac' teristics are displayed by the presentation.

HG. 3 illustrates the output of the-correlator of the invention with the diode feedback circuit providing a logging effect, so as to show the variable gain characteristics of the circuit. In FIG. 3 the gain in db is plotted as a function in time. In this FIG, the capacitor C is omitted. At an input signal level of 40db, the gain of the amplifier is a maximum and the output signal is quite noisy. It should be noted that a gain curve of the type shown in FIG. 3 generally does not change sign or polarity unless the polarity of one of the two input signals changes. Accordingly, in general, a curve of the type of FIG. 3 will only go in one direction showing either increasing or decreasing am. g Referring now to FIG. 4 there is shown a graph of the output voltage in millivolts as a function of time. The graph of FIG. d was obtained with the circuit of FIG. 1, including the capacitor C. Accordingly the circuit provides not only a variable gain, but a variable time constant, both varying proportional to the magnitude of the output signal. It can be seen from the FIG. that for low signal-to-noise ratios, the time constant of the amplifier is relatively long, thereby providing a slow output data rate.

The analogue signal correlator may be used for various purposes. For example, the two input signals A and B may be signals obtained by measuring the electric field and the magnetic field either in outer space or in the ionosphere. The purpose of the correlator is to determine whether, the two fields, namely the electric field and the magnetic field originate from a common electromagnetic field. In such a case there will be a correlation between the two signals at the same time. On the other hand, it is quite possible that the electric field was created by longitudinal plasma oscillations of the plasma which exists either in the ionosphere or further out in interplanetary space. In that case, only an electric field is created and there is no magnetic field. Accordingly, if two signals are correlated which originate in this manner, no correlation will be found.

It 18 also possible that large electric fields exist in the magnetosphere. in that region the apparent index of refraction may be very large, on the order of 150. In that case, an electromagnetic wave may exist which may create a magnetic field. However, the electric field is so much attenuated by the apparent large index of refraction that it cannot be measured.

Another example of a use of the analogue correlator of the invention is to compare two optical signals which have been derived from two separate photomultiplier tubes. It will be assumed, of course, that the signal is small and the signal-tonoise ratio a very small fraction. In this case the noise which is different for each of the tubes and for the electronics following the photomultiplier tubes may be very different even though the signals are the same. Therefore the circuit of the invention may be used to determine whether the two signals are the same in the presence of a large amount of noise.

In general the long-time constant available with the correlator of the invention makes it possible to integrate data with very large noise for a long period of time so that useable measurements may be made. On the other hand, where the signalto-noise ratio is large, a short-time constant suffices to integrate the signals to determine their correlation.

While there has been shown what is considered to be the preferred embodiment of the invention, it will be obvious that many changes and modifications may be made therein without departing from the essential spirit of the invention. For example: although diodes are shown as the nonlinear resistive element in the feedback path, it would also be possible to utilize other nonlinear elements such as junction transistors. It is intended, therefore, in the annexed claims, to cover all such changes and modifications as fall within the true scope of the invention.

I claim:

1. A signal correlator providing a substantially constant signal-to-noise ratio of two noiselike input signals to be correlated to determine, for example, a common signal origin, said correlator comprising:

a. a multiplier responsive to the two input signals to be correlated, said multiplier providing a multiplied signal; and

b. a nonlinear operational amplifier coupled to said multiplier for providing an output signal, said operational amplifier having a feedback path including a capacitor in parallel with a first set of serially connected diodes, and a second set of seriallyconnected diodes in parallel with said first set of diodes and saidcapacitor, said first set of diodes providing a first unidirectional current path, while said second set of diodes provides another unidirectional path opposite to said first current path, whereby the gain of said operational amplifier varies inversely proportional to the magnitude of said output signal.

2. A signal correlator providing a substantially constant signal-to-noise ratio of two noiselike input signals to be correlated, said correlator comprising:

a. a multiplier responsive to the two input signals to be correlated, said multiplier providing a product signal; and

b. a variable integrator comprising an operational amplifier coupled to said multiplier for providing an output signal, said operational amplifier including a feedback circuit for feeding back a portion of said output signal to the input of said operational amplifier, said feedback circuit comprising a capacitor, a first plurality of diodes serially connected, said first plurality of diodes in parallel with said capacitor and providing a first substantially unidirectional current path, and a second plurality of diodes serially connected, said second plurality of diodes in parallel with said capacitor and said first diodes and providing a second substantially unidirectional current path opposite to said first current path, whereby both the time constant and the gain of said amplifier inversely vary with the magnitude of said input signal. 

1. A signal correlator providing a substantially constant signal-to-noise ratio of two noiselike input signals to be correlatEd to determine, for example, a common signal origin, said correlator comprising: a. a multiplier responsive to the two input signals to be correlated, said multiplier providing a multiplied signal; and b. a nonlinear operational amplifier coupled to said multiplier for providing an output signal, said operational amplifier having a feedback path including a capacitor in parallel with a first set of serially connected diodes, and a second set of serially connected diodes in parallel with said first set of diodes and said capacitor, said first set of diodes providing a first unidirectional current path, while said second set of diodes provides another unidirectional path opposite to said first current path, whereby the gain of said operational amplifier varies inversely proportional to the magnitude of said output signal.
 2. A signal correlator providing a substantially constant signal-to-noise ratio of two noiselike input signals to be correlated, said correlator comprising: a. a multiplier responsive to the two input signals to be correlated, said multiplier providing a product signal; and b. a variable integrator comprising an operational amplifier coupled to said multiplier for providing an output signal, said operational amplifier including a feedback circuit for feeding back a portion of said output signal to the input of said operational amplifier, said feedback circuit comprising a capacitor, a first plurality of diodes serially connected, said first plurality of diodes in parallel with said capacitor and providing a first substantially unidirectional current path, and a second plurality of diodes serially connected, said second plurality of diodes in parallel with said capacitor and said first diodes and providing a second substantially unidirectional current path opposite to said first current path, whereby both the time constant and the gain of said amplifier inversely vary with the magnitude of said input signal. 